This invention relates to a solenoid valve and more particularly to a solenoid valve having an improved plunger response.
FIG. 1 is a sectional view showing a conventional solenoid valve to which the present invention pertains. In FIG. 1, the reference numeral 1 is a case, 2 is a magnetic plate, 3 is a plunger, 4 is a stationary iron core, 5 is a valve seat, 6 is a spring, 7 is a bobbin, 8 is a solenoid coil, 9 is a lead terminal of the solenoid coil, 10 is an O-ring, 11 is an inlet port and 12 is an outlet port. The case 1 is made of a magnetic material and serves as a magnetic yoke, the plate 2 is also made of a magnetic material, and the plunger 3 is made of a movable iron core. Thus, the case 1, the plate 2, the plunger 3, and the stationary iron core 4 together define a magnetic path.
When an electric current is passed through the coil 8, the coil 8 is energized to magnetically attract the plunger 3 against the action of the spring 6. When the coil 8 is deenergized, the stationary iron core 4 releases the plunger 3 so that the plunger 3 is moved forward by the spring 6.
Thus, the inlet port 11 is selectively opened and closed by the tip of the conical portion of the plunger 3 in response to the energization and deenergization of the coil 8. Therefore, when the valve is open, the control fluid flows in through the inlet port 11 and flows out through the outlet port 12, and when the valve is closed, the flow of the control fluid is interrupted.
As shown in FIG. 2, with the conventional solenoid valve, the fluid 13 flowing in through the inlet port 11 flows as shown by arrows, and magnetic foreign matter entrained in the fluid (iron particles, very fine ones which can pass through a filter) are magnetically attracted and accumulated on the surface of the conical portion of the plunger 3 as the coil 8 is repeatedly energized, and at the same time the accumulated particles are washed and moved by the flow of the fluid 13 into the clearance between the magnetic plate 2 and the plunger 3 and eventually, the magnetic foreign matter 14 builds up around the surfaces of the plunger 3. Such accumulated foreign matter 14 causes the characteristics of the solenoid valve to change. In particular, when the foreign matter 14 accumulates at the clearance between the end face of the plunger 3 and the stationary iron core 4 of a precision solenoid valve whose plunger stroke is about 0.5 mm, for example, the plunger stroke is disadvantageously affected by the accumulated foreign matter 14 to decrease the stroke to such an extent that it affects hydraulic control on the outlet port 12 and impedes the smooth and quick movement of the plunger 3.